The present invention relates to a soft handover method used in a CDMA mobile communication system.
In a cellular communication system, a mobile terminal is free to roam and connect from one base station to another within the mobile communication system. When a terminal is handling a call during a cell crossover, the call must also be switched to the new base station by a way which causes as little disturbance to the call as possible. The cell crossover process during an ongoing call is called a handover.
In a CDMA mobile communication system, a mobile terminal is able to communicate simultaneously with different base stations, these base stations being those registered in the active set of the mobile terminal. Each base station transmits data on a dedicated broadcast channel, e.g. on the broadcast channel called BCCH (Broadcast Control Channel) in the GSM system, this channel containing some information related notably to the load of this base station. The broadcast channels of the different base station of the CDMA system are sent with a same given power. Each mobile terminal of the network receives these different broadcast channels and demodulates them in order to determine a ratio Eb/I, where Eb is the energy level per bit received and I the spectral interference density (or Ec/I with Ec the energy level per chip) for each base station. This ratio corresponds to the quality with which the mobile terminal receives the broadcast channel of the corresponding base station. Alternatively, the received power level form the BCCH channel can be used as a quality criterion. The mobile terminal can therefore determine a signal quality level for each base station and can select the base stations that it receives with the best quality for soft handover purposes. This selection includes registering the best received base stations in its active set.
Such a process is performed on a regular time basis in order to ensure that the mobile terminal is always in communication with the best received base stations. Such communications with different base stations is called macrodiversity situation. This allows to lower the interference level of the system in comparison with a communication established with a single base station. Macrodiversity is also a means of improving signal quality for mobiles at cell border.
The goal of a soft handover criterion is to anticipate cell changes and detect potentially interesting macrodiversity situations. In this manner, interference is reduced in the network while maintaining quality.
Prior art solutions include downlink-oriented soft handover criteria. The mobile terminal carries out measurements (signal strength, Eb/I . . . ) over the downlink broadcast channel of each base station and compares the outcome of these measurements with fixed/dynamic thresholds or with the measurements realized on the broadcast channel of other base stations. Eventually, based on this criterion, the Radio Network Controller (Controller of the base stations) or the mobile terminal selects the base stations to be part of the mobile's active set, i.e. the base stations with which the mobile is simultaneously communicating. The active set contains preferably at least two or three base stations because the quality of the communications can change very rapidly and the active set is changed on a much lower basis.
However, with downlink-oriented handover criteria, situations can occur where uplink macrodiversity is wrongly unused (and therefore interference could be further minimized) because the handover criterion has not detected the need for macrodiversity in the uplink. Such a situation occurs when a base station that was not selected in the active set receives signals from a mobile terminal with a better quality than the base stations that are already in the active set. Since the uplink transmission path is not taken in consideration for building the active set, such a base station cannot be put in the active set, although the power of the signals sent by the mobile terminal could have been lowered.